Permanent magnets



y 0, 1960 M. DE BENNETOT I 2,936,408

' PERMANENT MAGNETS Filed Nov. 15, 1955 FIG- I FIG-2 United States Patent r PERMANENT MAGNETS Michel de Bennetot, Paris, France, assignor to Compagnie generale de Telegraphie Sans Fil, a corporation of rance Application November 15, 1955, Serial No. 546,993 Claims priority, application France November 18, 1954 6 Claims. (Cl. 317-201) The present invention relates to permanent magnets and, more particularly, to those designed to produce a substantially uniform magnetic field of high intensity in 2,936,408 Patented May 10, 1960 tively small compared to its length. It comprises essen tially: a first magnetic circuit having, for instance, the general shape of a U and an air-gap defined by two plane polar pieces, and a second magnetic circuit of generally tubular shape, surrounding, at least. partially, the air-gap of the first magnetic circuit. 7

According to a preferred embodiment of the invention, the second magnetic circuit is made up of an arrangement of elongated magnets, resulting for instance, from the sub-division, along the length of the air-gap, of a tubular magnet. Such tubular magnet may have a minimum transverse air-gap section in the middle of the airgap axis and increasing towards both ends of the magnet.

The invention will be better understood with the aid of the appended description and with reference to the annexed drawings, wherein: I

Fig. 1 diagrammatically shows one embodiment of a magnet according to the invention.

Fig. 2 diagrammatically shows a of a magnet according to In Fig. 1, a first magneti' circuit A, of U shape, comprises an air-gap l'bounded by the planar end faces of ends of the magnilavlnfiifis case, it can be considered that superficiaf'magnetic masses, located on the inner surface of the magnet, are more particularly concentrated in the neighbourhood of the middle portion of the air-gap, which produces a substantially uniform field over a consions. Such magnets are of particular interest especially for focusing the electron beam in travelling wave tubes.

Known tubular magnets, however, present a common drawback in that, as is well known, the magnetic field produced in the air-gap is about equal to the magnets demagnetizing field, and, consequently, is lower than the coercive force of the alloy of which it is made up. If it is desired to'produce, for instance, magnetic fields of the order of 600-650 oersteds, use can be made of alloys such as Alnico VI having a coercive force of 800 oersteds. desired to obtain, with tubular magnets, magnetic fields of higher intensities, of the order of 1500 oersteds, for instance.

Certain hard ferrites or certain manganese-bismuth alloys, with coercive forces attaining 2500-3000 oersteds, might be used. However, with such alloys, strong coercive fields are obtained only with very low magnetic permeabilities. For instance, the permeability of barium ferritcs is of the order of 1 to 1.25 for magnetic field strengths of about 800 to 1100 oersteds. Because of this lowpermeability, strong fields in the air-gap can only be obtained with tubular magnets of large cross-section which are bulkyand' difiicult to construct.

But the problem becomes diificult when it is,

Unlike tubularmagn'ets, conventional U-shaped magnets have the advantage of making it possible to obtain strong magnetic fields in t heair-gap, such values being often several times greater than thatof the demagnetizing field, and, hence, higher than the coercive force of the magnetic alloy constituting the magnet. But, the superficial magnetic masses are then localized at the ends of the air-gap, and it is difficult, therefore, to obtain a uniform field in the latter, except when its length is very small in proportion to its transverse dimensions.

It is an object of the present invention to provide a permanent magnet system, free of the above-mentioned disadvantages.

The magnet system according to the invention is capanetic field, while its transverse air-gap section is kept relatwo polar pieces 2 and 3. In practice, such a circuit may compriseone magnet or several magnets joined together or independent. Thiscircuit produces a field h in the air gap of the magnetic system according to the inven-' tion. Magnetic bars B B B form a second magnetic circuit surrounding and coaxial in the longitudinal direction with the air-gap of the first system and are parallel to the lines of force of the field h ,'thus producin in the aira of the ma netic s stem, a field h slderable length, Wlthout qui g a ge transverse dimeng g p g y 2 having the same direction as field h identical poles of the two circuits being located at approximately the same location along the longitudinal axis of the air-gap and on the same end of the air-gap of the composite magnet, as shown in the figure. In the magnetic system of Fig. l, the two constituent magnetic circuits mutually interact, thus combining their above-mentioned respective advantages, which result in a uniform field of high value being obtained. This can be roughly explained by considering that the tubular magnetic circuit provides a magnetic field passing within its tubular shape, the magnetic field being substantially radially symmetrical about the longitudinal axis about the air-gap of the U-shaped magnet, i.e. the field pattern being substantially symmetrical with respect to any plane which includes this longitudinal axis, in order to act uniformly from all sides of the air-gap to repel the lines of force of the U-shaped magnetic circuit and reduce their dispersion to the point of producing a uniform field in the air-gap, this field being a strongone since the U-shaped circuit generates an intense magnetic field. The demagnetizing fields in both magnetic circuits are stronger than they would have been, had they been isolated from oneanother; in other words, each has a tendency to demagnetize the other. Thus, while the coercive field of the alloy constituting the U-shaped circuit may be small, that of the alloy forming the tubular circuit must be large and greater than the uniform field which it is desired to set up in the air-gap, in order that the field of the U-shaped circuit shall not demagnetize the tubular circuit or reverse'its orientation.

Fig. 2 illustrates a longitudinal cross-sectional view of a magnet system according to the invention, comprising a magnetic circuit A and a tubular magnet B, the profile of which is such that the transverse section of the air-gap is a minimum in the neighbourhood of its middle portion, and increases towards the ends of the magnet.

The U-shaped magnetic circuit A is constituted by an alloy of high BH factor, so as to produce a strong field for small weight and bulk. Its dimensions are determined empirically in such a way that, after the magnet system is assembled, the alloy operates at its optimum op- W t h iggmi.

count being takenof its partial demagnetization by the second magnetic circuit. It is not necessary that said alloy shall have a very strong coercive field: AlIllCO V,

at the longitudinal axis passing through the middle of said air-gap and providing internally of its tubular shape a magnetic field which in its entirety is substantially symmetrical with respect to any plane which includes the for instance, may be employed. The alloy of ma gnetic longitudinal axis of said air-gap, said second permacircuit B has a strong coercive force, the value of which is greater than that of the uniform field to be set up in the air-gap. This condition is necessary in order that its magnetic orientation shall not be reversed when said nent magnet structure having a positive pole and a negative pole, the identical poles of the two permanent magnet structures being located at approximately the same location along the longitudinal axis of the air-gap andat circuit is placed in the neighbourhood of the air-gap of the same end of the air-gap, an air space being'provided magnetic circuit A. In the example illustrated in Figure 2, the air-gap has a length of 10 cm., the diameter of the polar pieces of the U-shaped magnet being 4 cm. The field which it is desired to set up in this air-gap is of 1200 oersteds. has a coercive force of about 650 oersteds, whereas the alloy forming the magnetic circuit B has a coercive force of the order of 1500 oersteds. The length of the U-shaped magnetic circuit is of the order of cm.

What I claim is:.

1. A permanent magnet system for producing a substantially uniform field of high value in an air-gap of greater length than transverse dimensions, comprising in combination, a first permanent magnet structure having between both said identical poles of said two structures at each end of said air-gap, the second magnet structure being made of a magnetic material having a higher coercive force than that of the first permanent magnet struc- Alnico V constituting the magnetic circuit A 15 ture.

5. A permanent magnet system comprising a first permanent magnet structure having opposed end poles defining therebetween an elongated air-gap, and a second permanent magnet structure being made of a magnetic material having a higher coercive force than that of the first permanent magnet structure, and laterally defining a substantially cylindrical air-gap at least part of which is common to said elongated air-gap and substantially coaxial therewith, said second permanent magnet structure provid-v an air-gap of greater longitudina, than transverse dimen- 5 ing Within said cylindrical air-gap a magnetic field which sions, and including a positive pole a danegative pole defining said air-gap, and a second permanen agnet structure of generally tubular shape substantially surrounding said air-gap having its axis substantially in alignment with the longitudinal axis of said air-gap and providing internally of its tubular shape amagnetic field which in its entirety is substantially symmetrical with respect to said longitudinal axis and to any plane which includes said longitudinal axis, said second permanent magnet structure having a positive pole and a negative pole, the like poles of both said permanent magnet structures being located at approximately the same location along the longitudinal axis of the air-gap and at the same end of the air-gap, the second permanent magnet structure being made of a magnetic material having a higher coercive force than that of the first permanent magnet srtucture, said two magnetic structures being separated at each end of the air-gap to provide an air space between said like poles at each end of said air-gap.

2. A permanent magnet system according to claim 1,

wherein the second permanent magnetic structure comprises a plurality of elongated magnets laterally bounding said air-gap of said first permanent magnet structure the transverse dimension of the longitudinal section of said air-gap being minimum at the middle portion of the axis thereof and increasing toward the ends of said second magnet structure.

3. A permanent magnet system according to claim 2 in which said first permanent magnet structure is U-shaped.

4. A permanent magnet system for producing a substantially uniform field of high value in an air-gap of greater length than transverse dimensions, comprising, in combination, a first permanent magnet structure having an air-gap of greater longitudinal than transverse dimensions, and including a positive pole and a negative pole defining said air-gap, and a second permanent magnet structure of generally tubular shape substantially surrounding said air-gap and symmetrically arranged about in its entirety is substantially symmetrical with respect to any plane which includes the longitudinal axis of said elongated air-gap, the poles of said two permanent magnet strfitures having a same denomination being situated at approximately the same location along thelongitudinal axis. of said elongated air-gap and at a same end of said elongated air-gap and an air space being provided therebetween thereby to provide a substantially uniform field of highvalue insaid elongated air-gap.

6. A permanent magnet system comprising a first permanent structure having opposed end poles defining therebetween an elongated air-gap, and a second permanent magnet structure being made of a magnetic material having a higher coercive force than that of the first permanent magnet structure, and laterally defining a substantially cylindrical air-gap at least part of which is superimposed on said elongated air-gap and substantially coaxial therewith, said second permanent magnet structure providing within said cylindrical air-gap a magnetic field which in its entirety is substantially symmetrical with respect to any plane which includes the longitudinal axis of said elongated air-gap, the poles of said two permanent magnet structures having a same denomination being situated at approximately the same location along the longitudinal axis of said elongated air-gap and at a same end of said elongated air-gap thereby to provide a substantially uniform field of high value in said elongated i -sap- References Cited in the file of this patent UNITED STATES PATENTS 1,362,008 Kane Dec. 14, 1920 2,398,653' Linlor Apr. 16, 1946 2,807,743 Ciofii Sept. 24, 1957 2,822,500 Bryant Feb. 4, 1958 FOREIGN PATENTS 1,105,964 France Dec. 9, 1 955 

